As the infant rat matures, suckling behaviors go through a transition and become less dependent on "built-in" schedules and more susceptible to motivational variables. We believe that milk is an important motivational stimulus for 10-14-day-old rat pups and think it likely that suckling responses are susceptible to being learned through response-contingent milk reinforcement. Because milk from the mother is always delivered contingent in suckling, but is not delivered on every response, the suckling-milk contingency for the rat is an intermittant one. The biologically-arranged partial reinforcement schedule might result in learned response persistence. The objectives of this research are to 1) conduct a comprehensive investigation of the structure and regulation of suckling and its relation to motivational and rhythmic arousal states, 2) ascertain whether suckling responses are susceptible to being learned as a natural consequence of nursing through response-contingent milk reinforcement, and 3) determine whether such learning results in response persistence which could affect weaning and adult behaviors.